This invention relates to the field of printhead cartridge bodies contained within inkjet printers. More particularly, this invention relates to a printhead cartridge body which includes a tape automated bonding (TAB) circuit having a plurality of apertures for connecting to a printed circuit board (PCB) and a printhead heater chip located on the printhead cartridge body.
Tape automated bonding (TAB) circuits are improved circuit mounting structures that are used to interconnect electronic elements in advanced microelectronic devices. TAB circuits can be used to electrically connect a semiconductor chip to a printed circuit board (PCB). Misalignment errors are common when connecting the TAB circuit to the PCB. Many TAB circuits are constructed having a polyimide layer enclosing the conductive traces, so visually inspecting the circuits for proper alignment is difficult. Furthermore, testing the circuit for open or short circuits can be a time consuming and expensive process since the electrical connections are not easily accessible. In prior art TAB circuits applications, misalignment errors were corrected by physically disassembling the connected circuits which usually destroyed one or both of the circuits.
Hot bar refers to the use of a heated metal or ceramic bar which is an efficient connection method used to form a solder joint between many electrical connectors at the same time. Hot bar is a technique that can be used to interconnect a flexible circuit to a PCB. Particularly, hot bar can be used to interconnect a PCB to a flexible TAB circuit.
In many hot bar applications, a flexible polyimide TAB circuit with conductive traces on one side of the circuit is connected to the electrical connectors of a PCB by transferring the heat from the hot bar through the polyimide and into the circuit trace. The transferred heat causes the PCB""s solder to flow, which electrically connects the PCB to the TAB circuit. Flexible TAB circuit designs sometimes utilize a small opening as a vent so that excess solder can flow away from the electrical connections. Since the traces of a TAB circuit are covered by polyimide, which acts as an insulator, the hot bar must transfer heat first through the polyimide layer before transferring heat to the underlying traces and PCB solder. By forcing heat transfer through a polyimide layer first, longer hot bar application times or higher heating temperatures are required, resulting in increased bonding time and production costs. The increased hot bar temperatures may also be detrimental to the polyimide material which may cause premature failure of the flex circuit.
Complications may occur if it is determined that an electrical connection within the PCB/TAB circuit configuration might be defective and the electrical connections need to be inspected. Since there is a polyimide surface covering the electrical connections, the entire circuit has to be removed to rework the electrical connections. Removing the circuit can destroy or significantly impair the electrical connectors of both the PCB and the TAB circuit. As a consequence, rework can be quite an expensive process.
TAB circuitry is useful since the traces and polyimide can be configured to the users specifications in a cost effective manner. However, because printhead cartridge body space is constrained by the area of printer, TAB circuit designs have been correspondingly limited. Therefore, improvements to the TAB circuit architecture are limited by the amount of TAB circuit surface area.
Adding to the design criteria of the TAB circuitry utilized within an inkjet printer is the ink contained within the printer. Ink is a highly corrosive contaminant to any exposed metal that is subjected to the ink. TAB circuitry containing exposed conductive metal traces will be subject to the highly corrosive ink within the inkjet printer and correspondingly the trace material can degrade after a short period of time, normally resulting in a short or open circuit.
What is needed therefore, is a TAB circuit which reduces hot bar processing times and temperatures, while enabling easy access to the electrical connections when the TAB circuit is being connected to a PCB. Furthermore, there is a need for preventing corrosion and deterioration of the electrical connections from ink mist within the inkjet printer.
With regard to the above, and other objects, the invention provides a printhead cartridge body for an inkjet printer which includes a TAB circuit having a unique architecture for improved manufacturing operations. Moreover, the TAB circuit architecture is readily sealable by a variety of methods.
The printhead cartridge body includes a TAB circuit having a first region, which further includes a first face and a second face. The first region of the TAB circuit also includes a first axis and a second axis that is substantially perpendicular to the first axis. The TAB circuit includes a second region, which contains a first face and a second face. The second region of the TAB circuit also includes a first axis and a second axis that is substantially perpendicular to the first axis.
A plurality of conductive traces are located on the second face of the first region and on the second face of the second region of the TAB circuit. Each trace has a first end and a second end. The first end of each trace is substantially parallel with the second axis of the first region of the TAB circuit. The second end of each trace is electrically connected to a printhead heater chip.
The first region of the TAB circuit contains at least two rectangular apertures. A length and a width define each aperture. A first axis extends along the length of each aperture and a second axis extends along the width of each aperture. The first axis of each aperture is aligned with the first axis of the first region of the TAB circuit. The second axis of each aperture is aligned with the second axis of the first region of the TAB circuit. An encapsulant layer is provided which substantially encloses the rectangular apertures preventing ink mist from penetrating into the apertures.
The printhead cartridge body also contains a PCB having a first face and a second face. The PCB includes a first axis and a second axis substantially perpendicular to the first axis. The PCB also includes a plurality of conductive contacts. The conductive contacts are electrically connected to the first ends of the conductive traces of the TAB circuit. The first face of the PCB is arranged adjacent to the second face of the first region of the TAB circuit. Furthermore, the first axis of the PCB is aligned substantially parallel with the first axis of the first region of the TAB circuit.
Additionally, the printhead cartridge body includes a printhead heater chip, having a first face and a second face. The printhead heater chip has a first axis and a second axis that is substantially perpendicular to the first axis. The printhead heater chip also includes a plurality of conductive contacts. The conductive contacts arc electrically connected to the second ends of the conductive traces of the TAB circuit. The first face of the printhead heater chip is arranged adjacent the second face of the second region of the TAB circuit. Moreover, the first axis of the printhead heater chip is aligned substantially parallel with the first axis of the second region of the TAB circuit.
The printhcad cartridge body including the TAB circuit with PCB region apertures as described above, is advantageous in a variety of ways. First, the hot bar cycle time period is much faster since heat does not have to be transferred through the polyimide layer, because the hot bar is applied directly to the exposed TAB circuit traces and solder on the conductive contacts of the PCB. Secondly, solder can flow around the entire TAB circuit lead providing a stronger junction between the TAB circuit and the PCB. In prior art systems, where KAPTON or analogous polymeric material covers one side of the lead, only the bottom and sides of the lead can come into contact with the solder, resulting in a weaker junction.
Furthermore, visual inspecting and reworking solder bridges and open circuits is no longer a major ordeal because the aperture locations allow easy access to the resulting circuit for repairs. For example, a KAPTON circuit that has no aperture or only vent holes will require that the entire circuit be removed to perform any kind of circuit repair. Removing the circuit normally destroys either the PCB, the flexible TAB circuit, or both.
Another advantage of the present invention is aperture orientation. The apertures are oriented on the TAB circuit so that the first axis of each aperture is aligned with the first axis of the first region of the TAB circuit. The second axis of each aperture is aligned with the second axis of the first region of the TAB circuit. By orienting the apertures on the TAB circuit as described above, the amount of printhead cartridge body space needed to accommodate the TAB circuit can be minimized. Furthermore, alignment error translation along the length of the TAB circuit is also effectively reduced.
By orienting the apertures in the above described fashion, chip/nozzle plate skew concerns are lessened. Chip/nozzle plate to TAB circuit skew is caused by angular misalignment of the chip/nozzle plate and the TAB circuit which translates along the length of the TAB circuit (i.e. alignment error translation), potentially creating a large misalignment error relative to the TAB circuit""s second (width) axis. The aperture orientation according to the invention, enables a fairly wide range of chip/nozzle plate to TAB circuit misalignment before serious degradation occurs and provides increased degrees of freedom with respect to alignment of the printhead and cartridge body components.
This invention also provides a method for sealingly connecting a FAB circuit having at least two apertures arranged in the circuit board region of the TAB circuit to a PCB. The method pertains to connecting the TAB circuit to a printhead heater chip and PCB in a printhead cartridge body for use in an inkjet printer.
The TAB circuit contains a printhead heater chip located in the printhead region of the TAB circuit. The TAB circuit is attached to an inkjet printer printhead cartridge body. The second face of the circuit board region of the TAB circuit is positioned adjacent to the first face of the PCB by substantially aligning the first axis of the circuit board region of the TAB circuit with the first axis of the PCB. A heating means is applied to traces disposed on the second face of the circuit board region of the TAB circuit through the apertures thereby fixedly connecting the traces of the TAB circuit to conductive contacts on the PCB. A sealant is applied to the apertures of the TAB circuit preventing ink from contacting and/or corroding the circuitry and connections.
The method described above provides several benefits not found in the prior art. First, hot bar application time is reduced since there is not a polyimide or similar type of surface covering the conductive traces of the TAB circuit. Therefore, heat transfer cycle time is reduced since the hot bar is directly applied to the conductive traces and corresponding conductive contacts. Secondly, the apertures allow greater misalignment flexibility when applying the TAB circuit to the PCB, which also reduces the application time since a high degree of precision is not required when mating the two components. Furthermore, the apertures allow visual inspection aiding in the alignment of the TAB circuit to the PCB.
Finally, sealant application is a simple process since the leads on the top and bottom of the circuit are exposed through the apertures. Also, with the design according to the invention, any conventional method of sealing can be used. Furthermore, scaling the apertures with a flowable encapsulant material adds mechanical strength to the enclosed bond between the conductive traces of the TAB circuit and the conductive contacts of the PCB. With prior art systems where apertures arc not provided, the soldered area has to be sealed from the sides or an adhesive/encapsulant has to be put down prior to use of the hot bar, neither method providing a cost effective sealing method.
For purposes of simplifying the description, the invention will be described with respect to use of TAB circuits for connecting between the printhead and PCB. However, it will be recognized by those of ordinary skill in the art that the features and advantages of the invention also apply to the use of flexible circuits for connecting between the printheads and PCB.